1. Field of the Invention
The field of the invention is that of synthetic three-dimensional cartographic representation represented in a conformal manner. Conformal representation refers to a cartographic representation that is superimposed on the terrain actually seen by the observer. The optical superimposition is provided by an optical system that projects the synthetic image towards the observer's eye. This optical system comprises an optical mixer or combiner which provides the superimposition of the image on the external landscape. This type of representation is particularly used in aeronautics in order to ensure piloting in poor visibility or at night.
2. Description of the Prior Art
A cartographic representation system is shown in FIG. 1. This system comprises a database BD of data representing the terrain flown over, means SN making it possible to determine the position and the orientation of the optical system displaying the cartographic image, an image generator CE and the optical projection system D. The image generator CE provides three main functions which are: the calculation of the point of view, the selection of the area of terrain to be displayed and the calculation of the image to be displayed as a function of the point of view.
One of the difficult points of this three-dimensional representation is that it must be sufficiently complete and accurate to give a good representation of the terrain flown over which can be useful for piloting and for navigation and sufficiently unobtrusive not to saturate the natural image of the landscape.
Various systems have been proposed. By way of example, the patents U.S. Pat. No. 8,264,498 entitled “System, apparatus, and method for presenting a monochrome image of terrain on a head-up display unit” and U.S. Pat. No. 7,098,913 entitled “Method and system for providing depth cues by attenuating distant displayed terrain” propose diverse approaches to this representation. A conventional approach consists in representing the terrain flown over in the form of a simple grid C as shown in FIG. 2 where the grid is shown in white lines on a black background. The perception of distance is produced solely by the size of the geometric elements of the grid which are delimited by the white lines in FIG. 2. In fact, under the effect of perspective, the longer the distance is from the displayed point of view, the smaller are the geometric elements on the screen. As seen in FIG. 2, this type of display naturally tends to saturate the parts most distant from the observer with lines whereas, in certain meteorological conditions, the distant parts of the terrain can be perfectly identifiable by the observer, notably in clear weather.